Corrosion is a natural process, and at the molecular level, involves a metal atom M being oxidized. The atom loses one or more electrons and leaves the bulk metal, M→Mm++me−. The lost electrons are conducted through the bulk metal to another site where they reduce (i.e. combine with) a reducible species such as a dissolved gas or a positively charged ion G+ that is in contact with the bulk metal, N+ne−→Nn− and Gm++me−→G.
The site where metal atoms lose electrons is called the anode, and the site where electrons are transferred to the reducible species is called the cathode. These sites can be located close to each other on the metal's surface, or far apart depending on the circumstances. When the anodic and cathodic sites are continuous, the corrosion is more or less uniform across the surface. When these sites are far apart, the anodic sites corrode locally.
A corrosion path is essentially an electric circuit, because there is a flow of current between the cathode and anode sites. In order for a current to flow, Kirchoff's circuit laws require that a circuit be closed and that there exists a driving potential (voltage). Part of the corrosion circuit is the base metal itself; the rest of the circuit exists in an external conductive solution (i.e. an electrolyte) in contact with the metal. This electrolyte takes away the oxidized metal ions from the anode and provides a reduction species (either nonmetalic atoms or metallic ions) to the cathode. Both the cathode and anode sites are immersed in an electrolyte for the corrosion circuit to be complete.
In corroding systems, potential gradients can be created by a number of mechanisms. These include differences in the free energy or the related electrochemical potentials for different reactions and gradients in the concentration of charged species in the solution. When two electrodes exhibiting differing potentials are electrically connected, a current flows in the external circuit.
There are various approaches to monitoring corrosion. Electrochemical approaches rely on the above-described electrochemical corrosion principles and the measurement of potentials or currents to monitor corrosion damage.
An emerging corrosion monitoring technology makes use of multi-electrode array sensors. Examples of such sensors are described in U.S. Pat. No. 6,132,593 to Tan, entitled “Method and Apparatus for Measuring Localized Corrosion and Other Heterogeneous Electrochemical Processes”, and in U.S. Pat. No. 6,683,463 to L. Yang, entitled “Sensor Array for Electrochemical Corrosion Monitoring”. These arrays may be used to study spatio-temporal patterns at corrosion sites on metals.